The potential of hydrogen (pH) measures the acidity or alkalinity of the soil on a scale from 0 to 14. A value of 7.0 is neutral, while numbers below 7.0 are acidic and those above 7.0 are alkaline. This chemical characteristic of the soil solution is a primary factor influencing plant health and growth. It dictates the environment in which plant roots absorb water and nutrients, determining whether essential elements are available or locked away from the plant.
How pH Dictates Nutrient Availability
Soil pH profoundly influences the solubility of minerals and the chemical form of nutrients, which plants absorb only when dissolved in the soil water. Most nutrients are optimally available within a slightly acidic to neutral range, between pH 6.0 and 7.0. Outside this optimal range, the concentration of hydrogen ions changes the chemical reactions that control nutrient availability.
In acidic conditions (below 5.5), certain micronutrients like iron, manganese, and zinc become highly soluble, potentially reaching toxic levels. Conversely, phosphorus, calcium, and magnesium become less available because they form insoluble compounds, often with aluminum. This nutrient lockup means the elements are physically present in the soil but inaccessible to the plant’s root system.
When the soil becomes alkaline (above 7.5), the reverse effect occurs, primarily impacting micronutrients. Elements such as iron, manganese, and zinc decrease significantly in solubility, often precipitating out of the soil solution as hydroxides and carbonates. This reduced availability causes deficiencies. Alkaline conditions, however, tend to increase the availability of calcium and magnesium.
Recognizing Symptoms of pH Imbalance
The physical appearance of a plant often displays symptoms of nutrient deficiency or toxicity, revealing an underlying pH problem. Since pH controls nutrient access, these visible signs are the first indication that the soil chemistry is out of balance.
A common sign of high pH is interveinal chlorosis, where the tissue between the leaf veins yellows while the veins remain green. This symptom frequently occurs in new growth and results from iron or manganese deficiency because these micronutrients are locked up in alkaline soil. In severely alkaline conditions, new leaves may appear almost white due to the lack of accessible iron.
Low pH conditions (below 5.5) can manifest as stunted root growth and poor plant vigor. This is linked to aluminum toxicity, which becomes highly soluble in acidic soil and damages the root structure. Additionally, the lack of available phosphorus in acidic soil can cause some plants to develop a dark green color with a reddish or purple tint on the underside of older leaves.
Practical Steps for Soil pH Management
The first step in managing soil pH is to accurately determine the current level using home testing kits or a professional soil testing laboratory. Lab tests provide a precise pH reading and often include recommendations for the amount of amendment needed based on the soil type and target pH. Different plants have different needs; most vegetables and lawns prefer a range of 6.0 to 7.0, while acid-loving plants like blueberries and rhododendrons thrive between 4.5 and 6.0.
To raise the pH of acidic soil, a process known as liming, several materials are commonly used. These materials work slowly and should be incorporated into the soil before planting for the most effective results.
- Agricultural lime contains calcium carbonate.
- Dolomite lime supplies both calcium and magnesium.
- Wood ash can also be used due to its high calcium content, though it is less effective than limestone and requires higher application rates.
Conversely, to lower the pH of alkaline soil, elemental sulfur is a frequently used amendment. Soil bacteria convert the sulfur into sulfuric acid, which gradually acidifies the soil. Adjusting soil pH is not a rapid process; it can take months for changes to fully stabilize, necessitating repeat testing to monitor progress. Other options include:
- Aluminum sulfate, a faster-acting option that should be used with caution.
- Canadian sphagnum peat moss, which can be incorporated to help reduce the pH, especially for acid-loving plants.